It is known that polyvinyl fluoride, polyvinylidene fluoride, and polyvinyl chloride, for example, may be made to possess enhanced piezoelectric and pyroelectric properties through the application of a dc electric field thereto at polarizing temperatures. In general, the application of a high dc voltage to a polar polymeric or thermoplastic film material will produce a film having enhanced electric properties. Care must be exercised however that dielectric breakdown of the film does not occur.
The present invention utilizes a corona discharge electrode roller which oscillates over the film to be poled which is transported by a slowly rotating drum, resulting in any given point or line on the film being subjected to a multiplicity of corona discharge passes by the corona discharge electrode. Thus, in lieu of a single high voltage discharge required by several prior art devices with frequent concomitant dielectric breakdown, the present invention employs much lower voltages resulting in substantially non-existent dielectric breakdown.
Additionally, many prior art polymer films require electroconductive coatings to be applied to the film surfaces prior to poling thereof, thus forming a capacitor-like device capable of storing high electric energy. Oftentimes the stored energy would discharge with resultant damage to the film. Even where a single electroconductive coating only is required, the added cost of its vacuum vaporizing on the film, for example, must be considered in any large scale production process. The present invention requires no electroconductive coating on either surface of the polymer film or sheet, although such a coating on one or both sides may be used.
Further, many prior art film poling apparatuses require the presence of heat while polarizing voltages are being applied thereto. The temperatures may range between about 50.degree. C. to slightly lower than the softening point of the film, or lower than approximately 180.degree. C. The elevated temperatures reduce the resistance of the film material enabling lesser voltages to effect an equivalent enhanced piezoelectricity. The present invention requires no elevated temperatures while the film is subjected to the corona poling voltages.
In brief, a field intensified ionization (corona) source of voltage repeatedly traverses the moving polarizable polymer in the present invention to result in fewer breakdowns of the polymer film, when compared to prior art static corona apparatuses, while yet providing a higher degree of operational flexibility, i.e., the frequency of the oscillating corona discharge electrode can readily be varied along with the speed of rotation of the drum which guides and transports the film thereover.